Hi tryemout and welcome to SaleHoo firstly, really nice to have you here.
Your question is one that we do see a lot, but there is never really a straight forward answer as different products perform differently across the board. Now there are of course those products that will sell well anywhere, but they are also usually the most popular products to sell, and that's of course makes them the most competitive!
The best possible place to start is selling unwanted things from around your home, it's a tried and proven method of getting started on a site such as eBay. Just a great way to test the market out with some products and also get yourself some experience and feedback under your belt as well.
If you enjoy selling second hand products like that, then you can start looking at avenues like garage sale (yard sales), weekend markets and that type of thing. These places can be an absolute gold mine for second hand goods, and 50-100 can get you plenty of stock to sell on eBay.
So many people thing that you have to be selling new items on eBay to make money, simply not true at all. eBay started life as a second hand marketplace and is still the best second hand market online anywhere.
There are some things you need to know to be successful with second hand goods, things like knowing the value of products (that is knowing the eBay value, not bricks and mortar value) It also helps if you can talk a good deal, sure the prices might already be cheap, but every dollar you can talk off a price is an extra dollar in your pocket.
If you haven't already, take a look at our Education Centre by selecting the Education Tab in the red bar above. This topic gets covered quite a bit on the forum, so if you have the time take a read through for ideas and advice as well.